STEP-compliant process planning and manufacturing

The aim of next generation Computer Numerically Controlled (CNC) machines is to be portable, interoperable and adaptable so that they can respond quickly to changes in market demand and the manufacturing needs of customized products. Over the years, G-codes (ISO 6983, RS 274 as known in the USA, or DIN 66025 as known in parts of Europe) have been extensively used by the CNC machine tools for part programming and are now considered as a bottleneck for developing next generation CNC machines. A new standard for STEP-compliant Computer Numerical Controllers, informally known as STEP-NC, is being developed as the data model for a new breed of CNCs. This data model represents a common standard specifically aimed at the intelligent CNC manufacturing workstation, making the goal of a standardized CNC controller and NC data generation facility a reality. It is believed that CNC machines implementing STEP-NC will be the basis for a more open and adaptable architecture. There are two parts to STEP-NC. The first is the Application Reference Model (ARM), i.e. ISO14649, which describes machining processes in terms of feature-based working steps. The second is the Application Interpreted Model (AIM), i.e. ISO 10303 AP238, which maps the ARM to the existing integrated resources of the full STEP standard. Despite the short history of STEP-NC and on-going development of the standards, a large amount of research work has already been carried out across different countries in the same region as well as on a truly international scale. CNC industry is now at a cross-road, where both CNC vendors and users are awaiting more compelling propositions and further convincing evidence for deploying and adopting STEP-NC in replacement of G-codes, or as the data exchange format with G-code treated as “internal” data for NC execution. It is the Guest Editors' intention that this special issue will provide a collective pool of knowledge about STEP-NC as well as exploratory showcases of implementing STEP-NC.

There are 14 articles included in this special issue, all pertaining to STEP-compliant process planning, manufacturing and computer numerical controls. Most of them are about direct deployment of a specific part of the STEP-NC standards for CNCs. The articles can be grouped into three categories, (a) Implementation of STEP-NC Data Models; (b) STEP-NC for Integrated and Distributed Manufacturing; and (c) New STEP-NC Data Models and Their Applications.

Implementation of STEP-NC data models

Six articles fall into this category. Four of them deal with milling applications whereas the other two deal with turning applications. Both ARM (ISO 14649) and AIM (ISO 10303 AP238) data models have been implemented.

The paper by Kramer et al. titled “Run-time interpretation of STEP-NC: implementation and performance” describes an experiment in building and running two STEP-NC interpreters, one using ISO 14649, the other using AP 238 of ISO 10303. Both use STEP Part 21 exchange files as input and call low-level machine control functions. Both include tool path generators and have sufficient functionality to execute the first example program at the end of Part 11 (milling) of ISO 14649. The discoveries have been instrumental, among which are (a) both ISO 14649 and AP 238 contain sufficient information to run a machining centre; (b) AP 238 faithfully remodels the information in ISO 14649 for the functionality that was implemented, and (c) real-time interpretation of both STEP-NC models is feasible.

The paper titled “On the futuristic machine control in a STEP-compliant manufacturing scenario” discussed a framework for an autonomous STEP-NC controller. The controller consists of two agents, planning agent and machining agent. Issues about part program interpretation and online process planning are also discussed. To achieve dynamic planning and the real-time requirements, the paper proposed to solve the on-line planning issues in two stages, i.e. the shop-floor planning and the real-time planning.

The work reported in “A framework and data processing for interfacing CNC with AP238” utilizes STEP-NC AIM models. The STEP-NC conceptual controller consists of an interpreting module (Interpreter), a planning module (Planner), a simulation module, and a CNC kernel. The Interpreter reads an AP238 file and converts it into internal data, which the Planner uses to sequence the specific machining operations and processes. The implementation tools and methods for interpreting an AP238 file are outlined through an example part and case study.

Hardwick and Loffredo in their paper, “STEP-NC AP-238 deployment”, described how AP-238 has been tested in the USA in preparation for deployment across the US machining industry. The tests showed that AP-238 allowed multiple CAM systems to send 5-axis tool path data to multiple CNC machines without any post-processing. The paper also describes the results of previous work performed on a feature based implementation of STEP-NC, and explains why industry has had difficulty in adopting a feature based implementation as its initial deployment method.

The paper titled “Integrating the CAx process chain for STEP compliant NC manufacturing of asymmetric parts” is one of the two articles on STEP-NC deployed for turning applications. It describes a methodology for implementing a standardized CAx process chain for rotational asymmetric parts that are described based on ISO 14649-12. The methodology is realized through a suite of STEP compliant NC software tools, based on STEP compliant information models to integrate the whole process chain from design to machining. Component case studies are used to demonstrate and analyse the methodology.

Choi et al. reported the other piece of turning work in their paper called “Development process and data management of TurnSTEP: A STEP-compliant CNC system for turning”. Based on the STEP-NC data model formalized as ISO 14649-12 and 121, TurnSTEP is designed to support intelligent and autonomous control of NC machines for e-Manufacturing. The developed TurnSTEP repository enables data sharing anytime, anywhere and on any platform. In addition, the paper presents a method to express STEP data using XML as a core technology of the repository. This enables product data to be easily stored and shared across the Web. A translator has been developed to convert STEP data in the clear text format into XML and vice versa.

STEP-NC for integrated and distributed manufacturing processes

With higher level information made available to a CNC controller, process integration and close-loop manufacturing can be made easier. STEP's Web-enabled feature gives STEP-NC an additional dimension in that distributed and collaborative manufacturing can be readily supported. There are four papers in this category; all have taken STEP-NC implementations one step beyond supporting pure CNC machining operations.

The paper “Enabling feedback of process data by use of STEP-NC” by Wosnik et al. presented a generic approach to pre-process and feed back process data from servo drives to CNCs and CAPP systems in order to enable higher level functionality of these systems. This approach rests on the application of open digital servo drives in machine tools and the design of application-dependent algorithms to process and exchange drive signals for both online and offline optimization of machining processes. Also discussed are the effects of the proposed methodology on design characteristics of open servo drives and CNC kernels as well as on the data interfaces. Communication protocols between their modules and CAPP systems are presented. The methodologies in this paper are exemplified by their application in drive-spanning process force reconstruction.

“Closed-loop CAPP/CAM/CNC process chain based on STEP and STEP-NC inspection tasks” by Brecher et al., introduces the integration of measuring technology into the STEP-NC based process chain, and in particular describes the current draft of ISO 14649 Part 16 – Data for touch probing based inspection – which allows the integration of inspection tasks into a sequence of machining operations. It gives a short overview of how Part 16 fits in with other inspection data exchange standards and draft standards, such as STEP AP219, DMIS (Dimensional Measuring Interface Standard), DML (Dimensional Markup Language) and I++DME (Dimensional Measuring Equipment). Furthermore, a prototype demonstration scenario for the closed-loop process chain is presented. It includes generation and execution of a STEP-NC program and feedback of measured results, achieved by automatically controlled STEP-NC inspection tasks, to the CAM system.

In the paper titled “Use of agents and neural networks for acquisition and preparation of distributed NC information to support NC planning”, Fichtner et al. describe applications of different intelligent methods in support of distributed technological data management in relation to STEP-NC. The paper suggests that the problem of current technological databases can be solved with the help of agent technology and machine learning for distributed information on shop-floor and planning departments. As a result, the authors introduced a combination of agent-based organization and self-learning of feature-based technological information.

Lee et al. in their paper titled “Development of PC based milling machine operated by STEP-NC in XML format”, introduced a format to use XML for representing STEP-NC. A 5-axis STEP-NC milling machine that is run by STEP-NC in XML format has been developed. The controller of this 5-axis STEP-NC milling machine can execute machining operations automatically. The interpreter for STEP-NC in XML format has a user-friendly interface. The system was tested using two XML files, one 2.5D and the other 3D. These two files were written using an XML editor from the developed interpreter. Both test files were machined automatically by the STEP-NC controller, free of G-code.

New STEP-NC data models and their applications

The authors of this group of articles described a number of extensions of STEP-NC to cover other application areas other than machining and inspection. The extendibility feature of STEP makes this possible. This is also necessary as it is desirable to have a uniform data structure for different manufacturing processes.

The first paper in this category is titled “Algorithms and an extended STEP-NC compliant data model for wire Electro Discharge Machining based on 3D representations”. This paper describes the STEP-NC data model and its implementation on wire Electro Discharge Machining (EDM). Wire EDM uses some unique working parameters, e.g. spark gap and speed of penetration. Most of these data are kept as company knowledge and based on experiences and historical data. STEP-NC helps to model this type of data. The current way of programming the wire's motion is to consider it as a 2.5 axis milling operations. The use of explicit ruled surfaces in STEP-NC allows real 3D offsetting and collision detection. The algorithms have been implemented in the STEP-NC environment for prototype software running on a wire EDM NC controller.

The paper, “Development of a data interface for Rapid Prototyping in STEP-NC” by Ryou et al., brings STEP-NC into Layered Manufacturing (LM), otherwise known as Rapid Prototyping (RP). Whereas a polygonal facet representation called stereolithography tessellation language (STL) has become the de facto industry standard for the transfer of geometric data to LM process planning systems, it has inevitable drawbacks such as redundancy, inaccuracy and lack of integrity. Furthermore, STL file format is not compatible with high level design information such as tolerance, mechanical properties and surface patterns, which will be indispensable for further design applications made by LM technologies. This paper proposed an EXPRESS data model, an information exchange foundation of modelling operation, for the new data transfer scheme in the LM community as one among STEP-NC (ISO 14649) modules. When using the proposed model data, the high level design information like machine dependent information for the LM process can be readily exchanged between different designers or LM users. This shared information between these people can greatly help LM engineers work better even though they are not original CAD designers or skilful equipment users.

Xie and Xu have attempted to develop a prototype STEP-compliant process planning system for sheet metal product development. This is reported in paper “A STEP-compliant process planning system for sheet metal parts”. It is noted that ISO 10303 AP207 was formally published by the ISO in 1999. However, it is deemed unsuitable for the applications targeted. The system has been designed based on a STEP-compliant product data structure, and has an interface to input CAD product data. It integrates software modules for nesting optimization, path optimization and planning, simulation, and machining parameters set up and CNC machining. The focus of the paper is placed on the structure of the prototype STEP-compliant system, as well as the definition of STEP-compliant product data models and interfaces for creating a STEP-compliant data environment, where the abovementioned software modules are integrated.

STEP-NC has also been implemented for manufacturing aesthetic products where less well-defined features are often present. This is reported in the paper titled “Strategy features for communicating aesthetic shapes for manufacturing” by Stroud and Xirouchakis. The specific application of this piece of work is to devise an approach to communicate machining features in stone products. Since stone products are usually aesthetic rather than functional, the feature set defined in STEP-NC (i.e. STEP AP-224) cannot be used directly. The “region_surface_list” feature type was used initially to communicate the basic shapes, but communicating aesthetic shapes using this method loses information about the basic shape to be made. It was therefore decided to use the “region_surface_list”, where appropriate, for communicating exact shape, augmented with an extra information layer to determine machining strategies. Other abnormities with aesthetic features include, relaxed tolerances, more precise information on material properties, and thin-wall situations where machining forces are of a particular concern. A new type of feature called “shell” has been introduced.

The above 14 papers are not to be considered as inclusive as far as STEP-NC research is concerned. The search for a complete integration of CAD/CAPP/CAM/CNC will continue; so will STEP-NC research. The Guest Editors would like to thank all the authors for their contributions. Many thanks go to the Editor-in-Chief, Prof. Stephen T. Newman for his great support and help. The editors are also grateful to the numerous reviewers who have helped shape the papers.